This invention relates to an arrangement for synchronizing the PWM-clock signals of several clocked direct voltage converters, which each comprise a control circuit having an individual oscillator, while each control circuit has a first connection for a capacitor and a second connection for a resistor for defining the frequency of the oscillator, the PWM-clock signal for driving the switching transistor being applied to a third connection and a pulse shifted in phase by 180.degree. with respect to the PWM-clock signal being applied to a fourth connection.
A circuit arrangement for synchronizing PWM-clock signals of several clocked direct voltage converters is known from DE-OS No. 3223179. In the known arrangement, the unit supplying a PWM-clock signal can be driven from the outside by a clock signal having a suitable frequency and a clock ratio of 1/2. The synchronization of the PWM-clock signals of several clocked direct voltage converters is therefore effected according to DE-OS No. 3223179 in a manner such that all of the direct voltage converters are supplied with the clock signal of the same clock generator. Since all the direct voltage converters are connected to the same primary voltage source--hereinafter this case is always assumed--, the synchronization of all the direct voltage converters leads in the manner described to high peak loads of the primary voltage source, for example during the first half cycle of the clock signal. In order to reduce the peak load, according to DE-OS No. 3223179 a part of all the direct voltage converters is supplied with the direct clock signal and another part is supplied with the inverted clock signal of the clock generator. Thus, the load on the primary voltage source is distributed substantially uniformly over the two half cycles of the clock signal. The PWM-clock signals thus synchronized of two arbitrary direct voltage converters then have either no phase shift (time shift) or a phase shift of 180.degree. with respect to each other.
When the individual direct voltage converters are equipped with commercially available driving elements--designated hereinafter as control circuits--, (more particularly the elements TDA 4714 and TDA 4716 are considered), problems arise because of the specific construction of these control circuits if several direct voltage converters are to be synchronized with a phase shift of 0.degree. or 180.degree. at choice.
The control circuits have two connections, one of which has to be provided with a resistor, while the other has to be provided with a capacitor, in order to define the frequency of the internal oscillator of the control circuit. If each control circuit was designed so that they all operate at the same nominal frequency, because of the mutually independent operations of the control circuits and of the direct voltage converters, the problem of the high peak load would be solved, it is true, but because of the inevitable deviations from the nominal frequency, modulation products would be formed, which would propagate over the lead to the primary voltage source and would disturb other loads connected to the primary voltage source. These modulation products would have to be suppressed by means of complicated filtering elements. Finally, it should be noted that the manufacturers of the control circuits indicated above have not provided any possibility for the synchronization of several control circuits.